Plural absorption stages for hydrogen purification

ABSTRACT

For the production of hydrogen, hydrocarbons are subjected to a partial oxidation, the resulting gaseous fraction is fed to a CO shift conversion and the acid gases are removed by absorption. To obtain a H2 product with a purity of at least 99% by volume without reduction in yield, sulfur compounds are removed in a first absorption stage with an organic, physical solvent; the resultant desulfurized gas is subjected to a selective catalytic oxidation with oxygen to selectively convert CO to CO2, and resultant desulfurized CO-depleted gas is treated in a second absorption stage with preferably the same scrubbing agent as first absorption stage to remove CO2.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation-in-part application of applicants' co-pendingapplication Ser. No. 620,786, filed June 15, 1984, entitled "PluralAbsorption Stages for Hydrogen Purification" now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a hydrogen purification process, andespecially to a process wherein hydrogen is produced from hydrocarbonsby partial oxidation, the resulting gaseous fraction is fed to a COconversion unit, and the acid gases are removed by scrubbing.

The production of hydrogen by partial oxidation of, e.g., fuel gases,fuel oils, residues or coal comprises the process steps of: (a)synthesis gas formation alternatively termed gasification; and (b) COshift conversion. In the gasification step, a gas consisting essentiallyof H₂, CO, CO₂, H₂ 0 and inert gases is obtained. The equilibriumconditions for the CO shift conversion generally are reached at a hightemperature, e.g. 1400° C. in the gasification reactor. When heavy oilis used, H₂ and CO are contained in the gas in about equal parts. Tomaintain the resultant equilibrium conditions, the crude gas is cooledrapidly by direct and/or indirect quenching. To raise the H₂ yield, theresultant gas is passed to the shift converter where it is saturatedwith steam and the CO therein is catalytically converted to CO₂ and H₂.Then the gas is cooled, the excess steam condensed out and the gas fedto the H₂ S removal and then to the CO₂ removal, which preferably isperformed by absorption with an organic, physical solvent. (A physicalsolvent is a solvent which does not enter into a chemical reaction withany components of the system). For additional details of the productionof hydrogen by partial oxidation, reference is invited to theliterature, e.g, Faith, Keyes and Clark's "Industrial Chemicals", FourthEdition, Lowenheim and Moran, John Wiley & Sons, N.Y. 1975, pages 470,471, 469, incorporated by reference herein.

By the process, a product with a purity of 97 to 98.5% hydrogen isobtained. With subsequent methanation, the CO and CO₂ content can bereduced to values less than 10 ppm, but at the expense of a loweredyield of H₂ which is consumed in the catalytic methanation reaction.Instead of methanation, a pressure swing adsorption installation can beused, with which a hydrogen purity of up to 99.99% by volume isachieved, but with about 10% of the amount of product lost. The pressureswing adsorption installation reaches its limits of economical use inthe case of high product pressures about 40 bar which, however, aredesirable in various cases, i.e. NH₃ -synthesis, hydrogenation anddesulfurization.

SUMMARY

An object of one aspect of this invention is to provide a hydrogenpurification process wherein an H₂ product with a purity of at least 99%by volume is produced, without any reduction in the yield of hydrogen.

An object of another aspect of this invention is to provide such apurification process in conjunction with the partial oxidation processfor the production of hydrogen.

Upon further study of the specification and appended claims, furtherobjects and advantages of this invention will become apparent to thoseskilled in the art.

To attain these objects, a process is provided wherein in a firstabsorption stage the sulfur compounds are removed with an organic,physical solvent, the resultant desulfurized gas is subjected to aselective catalytic CO oxidation, and the resultant gas is freed of CO₂with an organic, physical solvent in a second absorption stage. Ourinvestigations have shown that by a combination of an acid gasabsorption with a selective catalytic CO oxidation, a surprisingsynergistic effect occurs. Removal of the sulfur compounds before thecatalytic oxidation of CO to CO₂ is a prerequisite for use of a special,highly selective catalyst, which in the presence of oxygen makespossible the reaction

    CO+1/2O.sub.2 →CO.sub.2

while at the same time the reaction

    H.sub.2 +1/2O.sub.2 →H.sub.2 O

is reduced to a minimum. Thus the overwhelming part of the CO isconverted to CO₂, which is removed from the gas in the followingscrubbing stage. Such special highly selective catalysts have been usedby others in the U.S. and do not constitute a contribution of thisinvention. They are, e.g., Pt and Rh/Pd based catalysts with a carriermaterial of aluminium oxide, e.g., made by a proprietary manufacturingprocess. In particular, the "Selectoxo" catalyst, Engelhard IndustriesDivision of Engelhard Minerals & Chemicals Corp., is a catalyst that canbe used beneficially in the present invention; see Buckthorp, "AmmoniaPlant Productivity Boosted by Selective CO Oxidation", Nitrogen,May/June 1978, pp. 34-39.

A sulfur absorbing protective packing, e.g., iron oxide or zinc oxide,is advantageously installed upstream to protect the oxidation catalyst,the prefered arrangement is in a separate vessel according to standardengineering practice.

With the combination according to the invention of plural gas scrubbingsteps with a selective catalytic CO oxidation, an H₂ purity of over 99%by volume, especially up to 99.5% by volume, is obtained. This highproduct purity is fully satisfactory for most technical uses.

There are several alternatives for conducting the process of theinvention. According to one embodiment of the process according to theinvention, the sulfur compounds can be removed from the gas resultingfrom the CO shift conversion. Alternatively, it is possible to removethe sulfur compounds from the upstream synthesis gas resulting from thepartial oxidation, feed the desulfurized gas to the CO shift conversionand then feed the converted gas to the CO catalytic oxidation step.

It is advantageous to use the solvent resulting from the secondabsorption stage, charged with CO₂, to absorb the sulfur compounds inthe first absorption stage. Savings in regard to the regeneration ofthis solvent can be achieved by using a single absorption agent circuit.

Absorption of the sulfur compounds is performed with particularadvantage at temperatures below 0° C. The desulfurized gas is thenheated to ambient temperature and, after CO oxidation, is cooled to theabsorption temperature of below 0° C. in the second absorption stage.Thus, advantageously, the desulfurized gas from the first absorptionstage is heated in heat exchange with the gas to be desulfurized and,after selective CO oxidation, is again cooled in heat exchange with theH₂ product of the second absorption stage.

All physical absorption agents, which particularly exhibit a selectivityfor H₂ S as against CO₂, can be used as solvents in the processaccording to the invention. In particular, they are, among others,methyl alcohol, ketones, N-methylpyrrolidone, dimethylformamide,glycols, aromatic hydrocarbons, butyrolactone.

In general, the purification process of this invention is useful for thepurifying of any gas containing hydrogen, sulfur compounds and CO,whether obtained by partial oxidation or not, the volumetricconcentration of these components being generally about 50 to 90 %hydrogen, 0.1 to 5 % sulfur compounds and 0.2 to 2.0 % CO.

Depending on requirements, the gas freed of CO₂ can in a furtherembodiment be subjected to a methanation step to obtain higher purityproducts. In this case, the H₂ requirement for methanation is reduced bythe upstream catalytic CO oxidation and subsequent absorption of CO₂,which thereby leads to an improved product yield. Thus, with respect toH₂ yield, a pressure swing adsorption installation is about 10% poorerin yield than the alternative with methanation.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1 and 2 are block diagrams showing two embodiments for conductingthe process of the invention wherein FIG. 1 employs the sulfur compoundabsorption step after the shift converter, and FIG. 2 before the shiftconverter step; and

FIG. 3 is a schematic flowsheet of a preferred embodiment of the detailsof the absorption and regeneration steps of the invention.

DETAILED DESCRIPTION

According to FIG. 1, hydrocarbons are fed from pipe 1 to a synthesis gasunit 2. In the presence of oxygen, which, for example, is fed from anair fractionation unit 4 by pipe 3, the hydrocarbons are oxidized attemperatures of over 1200° C. essentially into H₂, CO and CO₂ which arewithdrawn via pipe 5.

The synthesis gas in pipe 5, is fed to a CO shift converter 7, in whichCO is catalytically reacted with H₂ 0 to form CO₂ and H₂ at temperaturesbetween 200 and 500° C., in one or more stages, the preferred catalystbeing based on Co/Mo sulfides.

The converted gas, essentially containing H₂, CO₂, steam, H₂ S and inertgases is passed via pipe 8 to a first absorption stage 9, in which thesulfur compounds, H₂ S and COS are selectively removed from the gas withan organic, physical solvent, and withdrawn via pipe 10. The resultantgas, thus desulfurized, is fed via pipe 11 to a selective catalytic COoxidation stage 12, which is supplied by pipe 13 with oxygen, the latterbeing provided optionally by air fractionation unit 4.

The gas, essentially free of CO, is then fed via pipe 14 to the secondabsorption stage 15, in which all the CO₂ is absorbed from the gas andremoved via pipe 16. The H₂ product is withdrawn via pipe 17.

According to FIG. 2, gas is fed by pipe 100 from a partial oxidation toa first absorption stage 101 and there is selectively freed of H₂ S andCOS with an organic, physical solvent. The sulfur compounds are removedvia pipe 102. The desulfurized gas is fed via pipe 103 to CO shiftconversion unit 104, which can consist of a high temperature and a lowtemperature stage. The water needed for CO conversion is fed by pipe105.

FIGS. 1 and 2 present established, process routes for gas treatmentdownstream petroleum oxidation units. They differ in the shift catalyst,which in FIG. 1 is sulfur resistant, consequently H₂ S and CO₂ areremoved in a one stage rectisol unit, with the intermediate catalyticstage.

In FIG. 2, H₂ S is removed in front of the shift catalyst and theoxidation step can be arranged immediately downstream the shift coolers.The preferred route is selected according to overall processrequirements independent from the catalytic oxidation step.

The converted gas is then fed via pipe 106 to selective, catalytic COoxidation 107, which is supplied with oxygen by pipe 108. The gas,essentially free of CO, is then fed via pipe 109 to the secondabsorption stage 110, in which all the CO₂ is removed from the gas andcarried off via pipe 111. The H₂ product stream is withdrawn via pipe112.

In FIG. 3, the process steps relating to the absorption process and COoxidation are illustrated in detail, using a rectisol absorptiontechnique as an example.

A typical cracked gas having a temperature of about 30° C. and apressure of 70 bars, resulting from the shift CO conversion and alreadyprecooled, is introduced via pipe 20. The cracked gas is mainlycomprised of H₂ and CO₂ but also contains CO, H₂ S and water. Thecracked gas is cooled to about -20° C. in a heat exchanger 21. Toprevent fouling of the apparatus with water ice, the crude gas isinjected with methyl alcohol by pipe 22 before cooling. The precooledcracked gas is then scrubbed for the removal of hydrogen sulfide in afirst absorption column 23. The H₂ S is scrubbed countercurrently in theabsorption column with cold methyl alcohol introduced at a temperatureof about -30° C. via pipe 24.

The desulfurized gas is passed via pipe 25 to heat exchanger 21 in whichit is heated to about ambient temperature. The heated gas is passed to acatalytic oxidation unit 26, in which CO is selectively oxidized to CO₂in the presence of oxygen introduced via conduit 27. The gas stream,essentially free of CO, is withdrawn from oxidation stage 26 via pipe 28and, after cooling to about -20° C. in a heat exchanger 29, is fed to asecond absorption column 30. In this column, the CO₂ is absorbed by coldmethyl alcohol (at about -50° C.) via pipe 31. An H₂ product streamhaving a temperature of -50° C. is withdrawn via pipe 32 from the headof the column, is heated in a heat exchanger 29 and is removed from theprocess via conduit 33. Optionally, this product stream can be furthertreated in a conventional methanation unit which is not shown. SeeSlack/James Amonia part II, pg. 311, Marcel Dekker, NY, 1974.

The methyl alcohol, charged with hydrogen sulfide from the firstabsorption column 23, is fed via pipe 34 and expansion valve 35 to aphase separator 36. By expansion to 25 bars in the expansion valve, mostof the H₂ also dissolved in the methyl alcohol is removed from thesolvent. The expanded gas is returned via pipe 37 and recycle compressor38 to crude gas stream 20 after cooling. The resultant methyl alcoholfrom phase separator 36 is delivered by pipe 39 into the middle of ahydrogen sulfide concentration column 40.

The methyl alcohol, charged with CO₂ from the second absorption column30 is withdrawn via pipe 41, and one part, about 40 to 60 %, afterpumping in pump 42 to the operating pressure of the first absorptioncolumn 23, is recycled thereto via pipe 24. The other part is fed viapipe 43 and expansion valve 44 to phase separator 45, which has the samepurpose as separator 36. The freed hydrogen is removed therefrom viapipe 46 and mixed with the hydrogen in pipe 37. The resultant methylalcohol, charged with CO₂ is delivered via pipe 47 into the upper partof the H₂ S concentration column.

Via pipe 48, into the bottom of the H₂ S concentration column 40, isintroduced a stripping gas, e.g., nitrogen, which functions to drive offthe CO₂ dissolved in the methyl alcohol. The resultant CO₂, togetherwith N₂, is withdrawn from the head of column 40 via pipe 49 as residualgas, thereafter heated in heat exchanger 29 to ambient temperature andthen withdrawn via conduit 50.

The methyl alcohol, charged with H₂ S and a residual CO₂ content, iswithdrawn from the bottom of column 40 via pipe 51 and introduced toregeneration column 52 after heating in heat exchanger 53. Regenerationcolumn 52 is provided with cooling 54 at the head and a closed heatingsystem 55 at the bottom. In regeneration column 52, the methyl alcoholis freed of H₂ S and CO₂. The H₂ S fraction is withdrawn via pipe 65 asa highly concentrated head product. The regenerated methyl alcohol ispassed through pipe 57 from the bottom of regeneration column 52 to heatexchanger 53 and from there by a pump 58 and pipe 31 to the secondabsorption column 30.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The following preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever. In the followingexamples, all temperatures are set forth uncorrected in degrees Celsius;unless otherwise indicated, all parts and percentages are by weight.

    ______________________________________                                        Partial oxidation of asphalt                                                  Nm.sup.3 h                                                                            1       2       3     4     5     6                                   ______________________________________                                        H.sub.2  62300  62000   61900 60380 61630 55800                               CO        500    500      50  --    --    --                                  Inert gases                                                                             230    230     230   260   310    5                                 CO.sub.2                                                                               35970    30      30        --    --                                  H.sub.2 S                                                                              1000   --      --    --    --    --                                  ______________________________________                                        Total   100000  62760   62210 61140 61940 55805                               ______________________________________                                        P[bar]  55      52      51    51    50    51                                  T[°C.]                                                                         40      30      30    40    40    30                                  Vol. % H.sub.2                                                                        62.3    98.8    99.5  98.8  99.5  99.99                               Yield % H.sub.2                                                                       100     99.5    99.4  96.9  98.9  89.6                                ______________________________________                                         1. Crude gas after 3stage conversion and cooling                              2. Crude H.sub.2 after acid gas absorption                                    3. Like 2, with catalytic CO oxidation                                        4. Pure H.sub.2 after methanation                                             5. Like 4, with catalytic CO oxidation                                        6. Pure H.sub.2 after purification in pressure swing adsorption               installation.                                                            

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

We claim:
 1. In a process for the production of hydrogen from sulfurcontaining hydrocarbons, wherein hydrocarbons are subjected to a partialoxidation to form a synthesis gas containing H₂, CO, CO₂, H₂ S and COS,the resulting gaseous fraction is subjected to a CO shift conversionwith steam, and the acid gases are removed by absorption, theimprovement which comprises removing sulfur compounds in a firstabsorption stage with an organic, physical solvent, subjecting thedesulfurized gas to a selective catalytic CO oxidation step with oxygen,the catalyst employed in said step being effective for the selectiveoxidation of CO to CO₂ in the presence of hydrogen, and then subjectingthe resultant desulfurized Co-depleted gas to a second absorption stagewith an organic, physical solvent to remove the CO₂ wherein the solventfrom the second absorption stage, charged with CO₂, is used partly forabsorption of the sulfur compounds in the first absorption stage.
 2. Aprocess according to claim 1, wherein the sulfur compounds are removedfrom the gas after the CO shift conversion step.
 3. A process accordingto claim 1, wherein the sulfur compounds are removed from the synthesisgas resulting from the partial oxidation and before the shift conversionreaction.
 4. A process according to claim 1, wherein the absorption ofthe sulfur compounds is performed at temperatures below 0° C., thedesulfurized gas is heated to ambient temperature and, after selectivecatalytic CO oxidation, cooled to the absorption temperature, below 0°C., of the second absorption stage.
 5. A process according to claim 2,wherein the absorption of the sulfur compounds is performed attemperatures below 0° C., the desulfurized gas is heated to ambienttemperature and, after selective catalytic CO oxidation, cooled to theabsorption temperature, below 0° C., of the second absorption stage. 6.A process according to claim 3, wherein the absorption of the sulfurcompounds is performed at temperatures below 0° C., the desulfurized gasis heated to ambient temperature and, after selective catalytic COoxidation, cooled to the absorption temperature, below 0° C., of thesecond absorption stage.
 7. A process according to claim 4, wherein thedesulfurized gas from the first absorption stage is heated in heatexchange with the gas to be desulfurized and, after selective COoxidation, again cooled in heat exchange with H₂ product from the secondabsorption stage.
 8. A process according to claim 5, wherein thedesulfurized gas from the first absorption stage is heated in heatexchange with the gas to be desulfurized and, after selective COoxidation, again cooled in heat exchange with H₂ product from the secondabsorption stage.
 9. A process according to claim 3, wherein thedesulfurized gas from the first absorption stage is heated in heatexchange with the gas to be desulfurized and, after selective COoxidation, again cooled in heat exchange with H₂ product from the secondabsorption stage.
 10. A process according to claim 1, wherein theresultant gas, freed of CO₂ is further subjected to a methanation stepto remove residual carbon oxides and further increase the purity of thehydrogen product.
 11. A process according to claim 1, wherein theorganic physical solvent is methanol.
 12. In a process for thepurification of hydrogen from a gas containing 50 to 90% hydrogen, 0.1to 5% sulfur compound and 0.2 to 2.0% CO, by volume, wherein the gas issubjected to a CO shift conversion with steam, and the acid gases areremoved by absorption, the improvement which comprises removing sulfurcompounds in a first absorption stage with an organic, physical solvent,subjecting the desulfurized gas to a selective catalytic CO oxidationstep with oxygen the catalyst employed in said step being effective forthe selective oxidation of CO to CO₂ in the presence of hydrogen, andthen subjecting the resultant desulfurized CO-depleted gas to a secondabsorption stage with an organic, physical solvent to remove the CO₂,wherein the solvent from the second absorption stage, charged with CO₂,is used partly for absorption of the sulfur compounds in the firstabsorption stage.